It has been known to provide a forming fabric in which so-called BE, or bulk-enhancing, yarns are periodically inserted into the PS weave; these yarns may be inserted at regular or irregular intervals and will account for from 10% to 50% of the PS weft yarns. The yarns have a vertical dimension that is at least 1.25 times greater than that of the “regular” weft yarns in the PS of the fabric. Two or more sets of these BE yarns can be used. The fabric construction may include at least one set of machine direction yarns, at least one set of cross-machine direction yarns, and an array of BE yarns. Such fabrics may have a multilayer construction and include pairs of intrinsic weft binder yarns, or may be a multilayer fabric whose warp yarns are ordered as triplet sets.
Multi-layer tissue forming fabrics are also known which are constructed to impart bulk in paper products formed thereon by providing a topographical difference between at least two yarns in the PS of the fabric. By using at least two different diameter, size or shape weft yarns positioned at the same contour in the forming surface it is possible to form pocket areas to increase surface area, develop topography and impart other desirable properties in tissue or towel products, for example.
Other papermaker's fabrics containing yarns of differing sizes in the papermaking surface are also known for various purposes with respect to increasing the life and runability of papermaker's fabrics on papermaking machines.
Sheet caliper and topography, thus bulk and absorbency, are highly desirable features of tissue and towel products. Paper machine clothing manufacturers strive to design fabrics which introduce differences in elevation (topography) in the papermaking surface of the fabric as it is these elevation differences which create pockets and protrusions in the paper product that in turn cause differences in fiber density in the sheet and increase its surface area. These factors amongst a number of others will help to provide an absorbent paper product.
Prior art fabrics currently in use have met with varying degrees of success. It has been recognized by the present inventor that larger pockets and protrusions in the paper product are more desirable (from a bulk and absorbency point of view) than a greater number of relatively smaller pockets and protrusions. It is also well known that sheet uniformity (i.e. a relatively even distribution of papermaking fibers) will provide a paper product with higher strength and other desirable physical properties. A problem common to prior art fabrics is that, in order to provide the desired bulking effect in the sheet, the fabric weave has been engineered so that it has relatively more “open” and “closed” areas; this leads to uneven drainage of the sheet and consequently uneven formation.
The present invention has noted these deficiencies and seeks to provide a fabric which is capable of producing larger pockets and thus develop caliper and augment topography and surface area at equal basis weights in the sheet product than has previously been possible, while maintaining good sheet uniformity and strength by retarding straight-through drainage in the fabric.